The present invention relates to apparatus for melting and thermally decomposing, for example, waste plastics and cooling the resulting thermal decomposition gas for condensation to thermal decomposition oil for recovery, and a process for melting and thermally decomposing waste plastics including polyvinyl chloride and cooling the resulting thermal decomposition gas for condensation to thermal decomposition oil for recovery.
Throughout the specification and the claims appended thereto, a treatment for removing HCl gas resulting from thermal decomposition from the system will be referred to as "desalting," and a chemical agent to be reacted with the HCl gas for the purpose of desalting and a device therefor will be referred to as a "desalting agent" and a "desalting column," respectively.
As one of such processes for thermally decomposing plastics, a process has been practiced which comprises melting and thermally decomposing plastics in a thermal decomposition reactor, feeding the resulting gas to a catalyst layer to decompose the gas into hydrocarbons of low carbon chain and thereafter condensing the hydrocarbons in a cooling-condensation unit to recover a light oil.
The plastics to be treated by such a process are generally those delivered by plastics manufacturers as industrial wastes and those discharged as classified refuse. Accordingly, these plastics, i.e., the material to be treated, includes metals, glasses and like extraneous matter as mixed therewith. During melting in the above process, the extraneous matter is released into time melt and therefore poses the problem that if plastics are continuously treated for thermal decomposition, the extraneous matter accumulates in the melt to fill up the reactor for melting and thermally decomposing plastics.
To solve this problem, accordingly, the apparatus is conventionally brought out of operation intermittently and cooled nearly to ambient temperature, and the accumulated extraneous matter is thereafter removed from the reactor.
However, the process involves a great energy loss in cooling the apparatus nearly to ambient temperature for the removal of extraneous matter and also a great energy loss as needed for heating to resume the operation after the removal, hence the problem exists of increased fuel consumption for the thermal decomposition.
The waste plastics delivered by plastics manufacturers as industrial wastes and the waste plastics discharged as separated refuse contain large quantities of waste polyvinyl chloride, so that if the waste plastics are collectively treated for thermal decomposition, decomposition of the polyvinyl chloride present produces HCl gas, which is likely to cause corrosion to the decomposition reactor, and the condenser and recovery unit which are subsequent to the reactor, further contributing to occurrence of pollution. Usually, polyvinyl chloride wastes are selectively removed roughly before thermal decomposition but invariably partly remain unremoved.
Already known among techniques for converting waste plastics into oil by thermal decomposition is a process for treating waste plastics including polyethylene, polypropylene, polyvinyl chloride, polyurethane, ABS resin, etc. by thermally decomposing these resins batchwise with addition of an alkali aqueous solution of high concentration under pressure of 10 kg/cm.sup.2 at a decomposition temperature of 400.degree. C. to 500.degree. C., and a batchwise two-stage thermal decomposition process involving a fixed catalyst phase-gas catalytic reaction for treating waste plastics as separated from motor vehicle shredder dust and common wastes and including polyethylene, polypropylene, polyvinyl chloride, ABS resin, etc., waste plastics being melted and dechlorinated in the first stage, and converted into oil by thermal decomposition in the second stage. The former process is practiced at a high pressure so as to hold water within the decomposition reactor and to recover low-boiling decomposition products only. The addition of the alkali aqueous solution results in expedited thermal decomposition but entails the drawback of giving an increased heavy oil fraction. With the latter process, the thermal decomposition oil is free from HCl gas produced, whereas measures need to be taken for preventing the decomposition gas recovery unit from corrosion with HCl gas.
The two processes described, however, are in the stage of basic experiments and have not been placed into actual use.